Reactions come everywhere in our environment, some reactions occurs very slow and some occurs very fast.  The area of chemistry that deals with the speed, or rate at which chemical reactions occur is called chemical kinetics.  The word "kinetic' means movement, motion or change.  Rates of reaction refers to the speed of chemical reaction, it is the change in the concentration of reactants or products per unit time.  Thus, the unit for reaction rate is molarity per seconds (M/s).  But before we discuss how to calculate the rate of reaction, let us have first the factors that affect rates of reaction.

Factors Affecting Reaction Rates

There are four factors that affects the rates of reaction, these are the physical state of the reactants, concentration of the reactants, temperature at which reaction occurs and the presence of catalyst.

1.  Physical state of the reactants.   Reactants need to collide in order to react.  The more the reactants collide with each other the faster is the reaction.  homogeneous reactions are more faster that heterogeneous reactions.  When reactants are in different phases, the reaction is limited to their area of contact that is why the reaction rate is slower compared to the same phases of reactants.  Reactions that involve solids tend to proceed faster if the surface area of solid is increased.  Like for example iodized salt dissolve faster in water than rock salt due to its large surface area.  The smaller the particle size of the substance the greater the surface area.

2.  The concentration of the reactants.  Increasing the concentration of the reactants increases the rate of reaction.  As concentration increases, the frequency of the molecules collision also increases, leading to increase rates.  For example, 70% alcohol solution is more concentrated than 40% alcohol solution, 70% alcohol solution is more effective in disinfecting your skin than 40% alcohol solution because of its high concentration.

3.  The temperature at which the reaction occurs.  The rate of chemical reaction increases when temperature is increased.   Just like for example sugar, sugar dissolve faster in hot water than in cold water.  Increasing the temperature increases the kinetic energy of molecules, thus molecules collide more frequently resulting faster rate of reaction.  This is also the reason why we refrigerate perishable foods like milk.

4.  The presence of catalyst.  Catalyst is a substance that is being added to the reactants in order that the reaction occurs faster and not being used up during the process. Enzymes for example act as our biological catalyst.  Enzymes increases the biological reaction in our body by factors ranging from  10⁶ to 10¹⁸.  Another example is adding papaya to our boiled chicken.  Papaya contains papain enzymes that increases the  rate of the tenderness of the chicken.

In general reactions can proceed faster or slower by controlling the different factors   If you want that reaction will go faster you can increase the reaction by increasing the surface area, increasing the concentration, increasing the temperature and adding catalyst to the reacting molecules. On the other hand if you want to decrease the rate of reaction you can do the opposite, decrease the surface area, decrease the concentration, decrease the temperature and add inhibitor instead of catalyst. Of course it will just depend on the given situation.  Avoiding for example the spoilage of food, food is placed inside the refrigerator not being left on the table or any container at room temperature.